JSON-LD Syntax 1.0

3.1 IRIs

Expressing IRIs are fundamental to Linked Data as that is how most subjects, all properties and many objects are identified. IRIs can be expressed in a variety of different ways in JSON-LD.

1. Except within a context definition, terms in the key position in a JSON object that have a mapping to an absolute IRI or another term in the active context are expanded to an IRI by JSON-LD processors.
2. An IRI is generated for the string value specified using @id or @type.
3. An IRI is generated for the string value of any key for which there are coercion rules in effect that identify the value as an @id.

IRIs may be represented as an absolute IRI, a relative IRI, a term, or a compact IRI.

An absolute IRI is defined in [RFC3987] containing a scheme along with path and optional query and fragment segments. A relative IRI is an IRI that is relative some other absolute IRI; in the case of JSON-LD this is the base location of the document.

IRIs can be expressed directly in the key position like so:

{
...
  "http://xmlns.com/foaf/0.1/name": "Manu Sporny",
...
}

In the example above, the key http://xmlns.com/foaf/0.1/name is interpreted as an IRI, as opposed to being interpreted as a string.

Term expansion occurs for IRIs if the value matches a term defined within the active context:

{
  "@context":
  {
    "name": "http://xmlns.com/foaf/0.1/name"
...
  },
  "name": "Manu Sporny",
...
}

Prefixes are expanded when the form of the value compact IRI represented as is prefix:suffix, and the prefix matches a term defined within the active context:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/"
...
  },
  "foaf:name": "Manu Sporny",
...
}

Terms are case sensitive, and must be matched using a case-sensitive comparison.

Keys that do not expand to an absolute IRI are ignored.

It is not determined if processing proceeds into values of undefined keys. If so, this would result in a graph which is not embedded.

foaf:name above will automatically expand out to the IRI http://xmlns.com/foaf/0.1/name. See Compact IRIs for more details.

An IRI is generated when a value is associated with a key using the @id keyword:

{
...
  "homepage": { "@id": "http://manu.sporny.org" }
...
}

Specifying a JSON Object with an @id key is used to identify that object using an IRI. This facility may also be used to link a subject with an object using a mechanism called embedding, which is covered in the section titled Embedding.

If type coercion rules are specified in the @context for a particular term or property IRI, an IRI is generated:

{
  "@context":
  {
    ...
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    }
    ...
  }
...
  "homepage": "http://manu.sporny.org/",
...
}

Even though the value http://manu.sporny.org/ is a string, the type coercion rules will transform the value into an IRI when processed by a JSON-LD Processor.

3.2 Identifying the Subject

To be able to externally reference nodes, it is important that each node has an unambiguous identifier. IRIs are a fundamental concept of Linked Data, and nodes should have a de-referencable identifier used to name and locate them. For nodes to be truly linked, de-referencing the identifier should result in a representation of that node. Associating an IRI with a node tells an application that the returned document contains a description of the node requested.

JSON-LD documents may also contain descriptions of other nodes, so it is necessary to be able to uniquely identify each node which may be externally referenced.

A subject of an object in JSON is declared using the @id key. The subject is the first piece of information needed by the JSON-LD processor in order to create the (subject, property, object) tuple, also known as a triple.

{
...
  "@id": "http://example.org/people#joebob",
...
}

The example above would set the subject to the IRI http://example.org/people#joebob.

To ensure the best possible performance, it is a best practice to put the @id keyword before other key-value pairs in an object. If it isn't listed first, processors have to save each key-value pair until @id is processed before they can start generating triples. Not specifying the @id keyword first creates a memory and complexity burden for one-pass processors.

3.3 Specifying the Type

The type of a particular subject can be specified using the @type keyword. Specifying the type in this way will generate a triple of the form (subject, type, type-IRI). To be considered Linked Data, types must be uniquely identified by an IRI.

{
...
  "@id": "http://example.org/people#joebob",
  "@type": "http://xmlns.com/foaf/0.1/Person",
...
}

3.4 Strings

Regular text strings, also referred to as string values, are easily expressed using regular JSON strings.

{
...
  "name": "Mark Birbeck",
...
}

3.5 String Internationalization

JSON-LD makes an assumption that strings with associated language encoding information are not very common when used in JavaScript and Web Services. Thus, it takes a little more effort to express strings with associated language information.

{
...
  "name": 
  {
    "@value": "花澄",
    "@language": "ja"
  }
...
}

The example above would generate a string value for 花澄 and associate the ja language code with the triple that is generated. Languages must be expressed in [BCP47] format.

3.6 Typed Values

A value with an associated type, also known as a typed value, is indicated by associating a value with an IRI which indicates the value's type. Typed values may be expressed in JSON-LD in three ways:

1. By utilizing the @type keyword when defining a term within a @context section.
2. By utilizing the expanded form for specifying objects.
3. By using a native JSON type.

The first example uses the @type keyword to express a typed value:

{
  "@context":
  {
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "modified":
    {
      "@id": "http://purl.org/dc/terms/modified",
      "@type": "xsd:dateTime"
    }
  }
...
  "modified": "2010-05-29T14:17:39+02:00",
...
}

The second example uses the expanded form for specifying objects:

{
...
  "modified":
  {
    "@value": "2010-05-29T14:17:39+02:00",
    "@type": "xsd:dateTime"
  }
...
}

Both examples above would generate an object with the value of 2010-05-29T14:17:39+02:00 and the type of http://www.w3.org/2001/XMLSchema#dateTime.

The third example uses a built-in native JSON type, a number, to express a type:

{
...
  "@id": "http://example.org/people#joebob",
  "age": 31
...
}

The example above is really just a shorthand for the following:

{
...
  "@id": "http://example.org/people#joebob",
  "age":
  {
    "@value": "31",
    "@type": "http://www.w3.org/2001/XMLSchema#integer"
  }
...
}

The @type keyword is also used to associate a type with a subject. Although the same keyword is used in both places, the concept of an object type and a value type are different. This is similar to object-oriented programming languages where both scalar and structured types use the same class inheritance mechanism, even though scalar types and structured types are inherently different.

3.7 Multiple Objects for a Single Property

A JSON-LD author can express multiple values in a compact way by using arrays. If a subject has multiple values for the same property, the author may express each property as an array.

In JSON-LD, multiple objects on a property are not ordered. This is because graphs are inherently unordered data structures. To learn more about creating ordered collections in JSON-LD, see the section on Lists.

{
...
  "@id": "http://example.org/people#joebob",
  "nick": [ "joe", "bob", "jaybee" ],
...
}

The markup shown above would generate the following triples:

<http://example.org/people#joebob>
   <http://xmlns.com/foaf/0.1/nick>
      "joe" .
<http://example.org/people#joebob>
   <http://xmlns.com/foaf/0.1/nick>
      "bob" .
<http://example.org/people#joebob>
   <http://xmlns.com/foaf/0.1/nick>
      "jaybee" .

3.8 Multiple Values for a Single Property

Multiple values may also be expressed using the expanded form for objects:

{
  "@id": "http://example.org/articles/8",
  "dc:title": 
  [
    {
      "@value": "Das Kapital",
      "@language": "de"
    },
    {
      "@value": "Capital",
      "@language": "en"
    }
  ]
}

The markup shown above would generate the following triples:

<http://example.org/articles/8>
   <http://purl.org/dc/terms/title>
      "Das Kapital"@de .
<http://example.org/articles/8>
   <http://purl.org/dc/terms/title>
      "Capital"@en .

3.9 Lists

Because graphs do not describe ordering for links between nodes, in contrast to plain JSON, multi-valued properties in JSON-LD do not provide an ordering of the listed objects. For example, consider the following simple document:

{
...
  "@id": "http://example.org/people#joebob",
  "nick": [ "joe", "bob", "jaybee" ],
...
}

This results in three triples being generated, each relating the subject to an individual object, with no inherent order.

As the notion of ordered collections is rather important in data modeling, it is useful to have specific language support. In JSON-LD, a list may be represented using the @list keyword as follows:

{
...
  "@id": "http://example.org/people#joebob",
  "foaf:nick":
  {
    "@list": [ "joe", "bob", "jaybee" ]
  },
...
}

This describes the use of this array as being ordered, and order is maintained through alternate representations as described in [JSON-LD-API]. If every use of a given multi-valued property is a list, this may be abbreviated by setting @container to @list in the context:

{
  "@context":
  {
    ...
    "nick":
    {
      "@id": "http://xmlns.com/foaf/0.1/nick",
      "@container": "@list"
    }
  },
...
  "@id": "http://example.org/people#joebob",
  "nick": [ "joe", "bob", "jaybee" ],
...
}

List coercion is specified within an expanded term definition using the @container key. The value of this key, if present, must be @list. This indicates that array values of keys coerced as @list are to be serialized as a List.

List of lists are not allowed in this version of JSON-LD. If a list of lists is detected, a JSON-LD processor will throw an exception.

4.1 Compact IRIs

Terms in Linked Data documents may draw from a number of different vocabularies. At times, declaring every single term that a document uses can require the developer to declare tens, if not hundreds of potential vocabulary terms that are used across an application. This is a concern for at least three reasons; the first is the cognitive load on the developer of remembering all of the terms, the second is the serialized size of the context if it is specified inline, the third is future-proofing embedded application contexts that may not be easy to change after they are deployed. In order to address these issues, the concept of a compact IRI is introduced.

A compact IRI is a way of expressing an IRI using a prefix and suffix. Generally, these prefixes are used by concatenating the prefix and a suffix, which is separated by a colon (:). The prefix is a term taken from the active context and is a short string identifying a particular IRI in a JSON-LD document. For example, the prefix foaf may be used as a short hand for the Friend-of-a-Friend vocabulary, which is identified using the IRI http://xmlns.com/foaf/0.1/. A developer may append any of the FOAF vocabulary terms to the end of the prefix to specify a short-hand version of the absolute IRI for the vocabulary term. For example, foaf:name would be expanded out to the IRI http://xmlns.com/foaf/0.1/name. Instead of having to remember and type out the entire IRI, the developer can instead use the prefix in their JSON-LD markup.

Terms are interpreted as compact IRIs if they contain at least one colon and the first colon is not followed by two slashes (//, as in http://example.com). To generate the full IRI, the value is first split into a prefix and suffix at the first occurrence of a colon (:). If the active context contains a term mapping for prefix, an IRI is generated by prepending the mapped prefix to the (possibly empty) suffix using textual concatenation. If no prefix mapping is defined, the value is used directly as an absolute IRI. If the prefix is an underscore (_), the IRI remains unchanged. This effectively means that every term containing a colon will be interpreted by a JSON-LD processor as an IRI.

The ability to use compact IRIs reduces the need for developers to declare every vocabulary term that they intend to use in the JSON-LD context. This reduces stand-alone JSON-LD document serialization size because every vocabulary term need not be declared in the embedded context. Compact IRIs also reduces the cognitive load on the developer. It is far easier to remember foaf:name than it is to remember http://xmlns.com/foaf/0.1/name. The use of prefixes also ensures that a context document does not have to be updated in lock-step with an externally defined vocabulary. Without prefixes, a developer would need to keep their application context terms in lock-step with an externally defined vocabulary. Rather, by just declaring the vocabulary prefix, one can use new terms as they're declared without having to update the application's JSON-LD context.

Consider the following example:

{
  "@context":
  {
    "dc": "http://purl.org/dc/elements/1.1/",
    "ex": "http://example.org/vocab#"
  },
  "@id": "http://example.org/library",
  "@type": "ex:Library",
  "ex:contains":
  {
    "@id": "http://example.org/library/the-republic",
    "@type": "ex:Book",
    "dc:creator": "Plato",
    "dc:title": "The Republic",
    "ex:contains":
    {
      "@id": "http://example.org/library/the-republic#introduction",
      "@type": "ex:Chapter",
      "dc:description": "An introductory chapter on The Republic.",
      "dc:title": "The Introduction"
    }
  }
}

In this example, two different vocabularies are referred to using prefixes. Those prefixes are then used as type and property values using the compact IRI prefix:suffix notation.

It's also possible to use compact IRIs within the context as shown in the following example:

{
  "@context":
  {
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "foaf": "http://xmlns.com/foaf/0.1/",
    "foaf:homepage": { "@type": "@id" },
    "picture": { "@id": "foaf:depiction", "@type": "@id" }
  },
  "@subject": "http://me.markus-lanthaler.com/",
  "@type": "foaf:Person",
  "foaf:name": "Markus Lanthaler",
  "foaf:homepage": "http://www.markus-lanthaler.com/",
  "picture": "http://twitter.com/account/profile_image/markuslanthaler"
}

Compact IRIs, also known as CURIEs, are defined more formally in RDFa Core 1.1, Section 6 "CURIE Syntax Definition" [RDFA-CORE]. JSON-LD does not support the square-bracketed CURIE syntax as the mechanism is not required to disambiguate IRIs in a JSON-LD document like it is in HTML documents.

4.2 External Contexts

Authors may choose to declare JSON-LD contexts in external documents to promote re-use of contexts as well as reduce the size of JSON-LD documents.

In order to use an external context, an author must specify an IRI to a valid JSON-LD document. The referenced document must have a top-level JSON Object. The value of any @context key within that object is substituted for the IRI within the referencing document to have the same effect as if the value were specified inline within the referencing document.

The following example demonstrates the use of an external context:

{
  "@context": "http://json-ld.org/contexts/person",
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
}

Authors may also import multiple contexts or a combination of external and local contexts by specifying a list of contexts:

{
  "@context":
  [
    "http://json-ld.org/contexts/person",
    {
      "foaf": "http://xmlns.com/foaf/0.1/"
    },
    "http://json-ld.org/contexts/event",
  ]
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
  "celebrates":
  {
    "@type": "Event",
    "description": "International Talk Like a Pirate Day",
    "date": "R/2011-09-19"
  }
}

Each context in a list will be evaluated in-order. Duplicate mappings among the contexts must be overwritten on a last-defined-overrides basis. The context list must contain either de-referenceable IRIs or JSON Objects that conform to the context syntax as described in this document.

An author may nest contexts within JSON objects, with the more deeply nested contexts overriding the values in previously defined contexts:

{
  "@context":
  {
    "name": "http://example.com/person#name",
    "details": "http://example.com/person#details"
  },
  "name": "Markus Lanthaler",
  ...
  "details":
  {
    "@context": { "name": "http://example.com/organization#name" },
    "name": "Graz University of Technology"
  }
}

In the example above, the name prefix is overridden in the more deeply nested details structure. Note that this is rarely a good authoring practice and is typically used when the JSON object has legacy applications using the structure of the object.

External JSON-LD context documents may contain extra information located outside of the @context key, such as documentation about the prefixes declared in the document. When importing a @context value from an external JSON-LD context document, any extra information contained outside of the @context value must be discarded. It is also recommended that a human-readable document encoded in HTML+RDFa [HTML-RDFA] or other Linked Data compatible format is served as well to explain the correct usage of the JSON-LD context document.

4.3 Referencing Contexts from JSON Documents

Ordinary JSON documents can be transformed in JSON-LD documents by referencing to an external JSON-LD context in an HTTP Link Header. Doing this allows JSON to be unambiguously machine-readable without requiring developers to drastically change their workflow and provides an upgrade path for existing infrastructure without breaking existing clients that rely on the application/json media type.

In order to use an external context with an ordinary JSON document, an author must specify an IRI to a valid JSON-LD document in an HTTP Link Header [RFC5988] using the describedby link relation. The referenced document must have a top-level JSON Object. The @context subtree within that object is added to the top-level object of the referencing document. If an array is at the top-level of the referencing document and its items are objects, the @context subtree is added to all array items. All extra information located outside of the @context subtree in the referenced document must be discarded.

The following example demonstrates the use of an external context with an ordinary JSON document:

GET /ordinary-json-document.json HTTP/1.1
Host: example.com
Accept: application/json,*/*;q=0.1

----------------------------------------------------

HTTP/1.0 200 OK
...
Content-Type: application/json
Link: <http://json-ld.org/contexts/person>; rel="describedby"; type="application/ld+json"

{
  "name": "Markus Lanthaler",
  "homepage": "http://www.markus-lanthaler.com/",
  "depiction": "http://twitter.com/account/profile_image/markuslanthaler"
}

JSON-LD documents must have all context information, including references to external contexts, within the body of the document.

4.4 Default Language

JSON-LD allows a default value to use as the language for string values. It is commonly the case that documents are written using a single language. As described in String Internationalization, a language-tagged value may be specified as follows:

{
  ...
  "name":
  {
    "@value": "花澄",
    "@language": "ja"
  }
}

It is also possible to apply a particular language code to all string values by setting the @language key in the @context:

{
  "@context":
  {
    ...
    "@language": "ja"
  },
  "name": "花澄",
  "occupation": "科学者"
}

The example above would generate a string value for 花澄 and 科学者 and associate the ja language code with each value.

It is possible to override the default language by using the expanded form of a value:

{
  "@context":
  {
    ...
    "@language": "ja"
  },
  "name": "花澄",
  "occupation":
  {
    "@value": "Scientist",
    "@language": "en"
  }
}

It is also possible to override the default language and specify a plain value by omitting the @language tag when expressing the expanded value:

{
  "@context":
  {
    ...
    "@language": "ja"
  },
  "name": "花澄",
  "occupation": 
  {
    "@value": "Ninja"
  }
}

Object properties that use the expanded form are considered explicitly defined. The @language keyword, when used in the context, must only be applied to string values. That is, string values expressed in expanded form are not affected by the @language keyword, when it is used in the context.

To clear the default language for a subtree, @language can be set to null in a local context as follows:

{
  "@context":
  {
    ...
    "@language": "ja"
  },
  "name": "花澄",
  "details":
  {
    "@context":
    {
      "@language": null
    },
    "occupation": "Ninja"
  }
}

4.5 Expanded Term Definition

Within a context definition, terms may be defined using an expanded notation to allow for additional information associated with the term to be specified (see Type Coercion and Lists).

Instead of using a string representation of an IRI, the IRI may be specified using an object having an @id key. The value of the @id key must be either a prefix:suffix value, an IRI. Type information may be specified

{
  "@context":
  {
    "foaf": { "@id": "http://xmlns.com/foaf/0.1/" },
    "name": { "@id": "http://xmlns.com/foaf/0.1/name" },
    "homepage": { "@id": "foaf:homepage" },
    "depiction": { "@id": "foaf:depiction" }
  },
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/",
  "depiction": "http://twitter.com/account/profile_image/manusporny"
}

4.6 Automatic Typing

Since JSON is capable of expressing typed information such as doubles, integers, and boolean values. As demonstrated below, JSON-LD utilizes that information to create typed values:

{
...
  // The following two values are automatically converted to a type of xsd:double
  // and both values are equivalent to each other.
  "measure:cups": 5.3,
  "measure:cups": 5.3e0,
  // The following value is automatically converted to a type of xsd:double as well
  "space:astronomicUnits": 6.5e73,
  // The following value is never converted to a language-native type
  "measure:stones": { "@value": "4.8", "@type": "xsd:decimal" },
  // This value is automatically converted to having a type of xsd:integer
  "chem:protons": 12,
  // This value is automatically converted to having a type of xsd:boolean
  "sensor:active": true,
...
}

When dealing with a number of modern programming languages, including JavaScript ECMA-262, there is no distinction between xsd:decimal and xsd:double values. That is, the number 5.3 and the number 5.3e0 are treated as if they were the same. When converting from JSON-LD to a language-native format and back, type information is lost in a number of these languages. Thus, one could say that 5.3 is a xsd:decimal and 5.3e0 is an xsd:double in JSON-LD, but when both values are converted to a language-native format the type difference between the two is lost because the machine-level representation will almost always be a double. Implementers should be aware of this potential round-tripping issue between xsd:decimal and xsd:double. Specifically objects with a type of xsd:decimal must not be converted to a language native type.

4.7 Type Coercion

JSON-LD supports the coercion of values to particular data types. Type coercion allows someone deploying JSON-LD to coerce the incoming or outgoing types to the proper data type based on a mapping of data type IRIs to property types. Using type coercion, value representation is preserved without requiring the data type to be specified with each usage.

Type coercion is specified within an expanded term definition using the @type key. The values of this key represent type IRIs and must take the form of term, compact IRI, absolute IRI or the keyword @id. Specifying @id indicates that within the body of a JSON-LD document, string values of keys coerced as @id are to be interpreted as IRIs.

Terms or compact IRIs used as the value of a @type key may be defined within the same context.

The example below demonstrates how a JSON-LD author can coerce values to typed values, IRIs and lists.

{
  "@context":
  {
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "http://xmlns.com/foaf/0.1/name",
    "age":
    {
      "@id": "http://xmlns.com/foaf/0.1/age",
      "@type": "xsd:integer"
    },
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id",
      "@container": "@list"
    }
  },
  "name": "John Smith",
  "age": "41",
  "homepage":
  [
    "http://personal.example.org/",
    "http://work.example.com/jsmith/"
  ]
}

The example above would generate the following Turtle:

@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

[ foaf:name "John Smith";
  foaf:age  "41"^^xsd:integer;
  foaf:homepage ( <http://personal.example.org/> <http://work.example.com/jsmith/> )
] .

Terms may also be defined using absolute IRIs or compact IRIs. This allows coercion rules to by applied to keys which are not represented as a simple term. For example:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "foaf:age":
    {
      "@id": "http://xmlns.com/foaf/0.1/"",
      "@type": "xsd:integer"
    },
    "foaf:homepage":
    {
      "@type": "@id"
    }
}

In this case, the @id definition is optional, but if it does exist, the compact IRI or IRI is treated as a term so that the actual definition of a prefix becomes unnecessary.

4.8 IRI Expansion Within a Context

To be consistent with JSON-LD, in general, anywhere an IRI is expected, normal IRI expansion rules apply (see IRIs). Within a context definition, this can mean that terms defined within a given context may also be used within that context, as long as there are no circular dependencies. For example, it is common to use the xsd namespace when defining typed values:

{
  "@context":
  {
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "http://xmlns.com/foaf/0.1/name",
    "age":
    {
      "@id": "http://xmlns.com/foaf/0.1/age",
      "@type": "xsd:integer"
    },
    "homepage":
    {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    }
  },
  ...
}

In this example, the xsd term is defined, and used as a prefix for the @type coercion of the age property.

Terms may also be used when defining the IRI of another term:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "foaf:name",
    "age":
    {
      "@id": "foaf:age",
      "@type": "xsd:integer"
    },
    "homepage":
    {
      "@id": "foaf:homepage",
      "@type": "@id"
    }
  },
  ...
}

Terms may also be used on the left-hand side of a definition.

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "foaf:name",
    "foaf:age":
    {
      "@id": "foaf:age",
      "@type": "xsd:integer"
    },
    "foaf:homepage":
    {
      "@type": "@id"
    }
  },
  ...
}

Note that in this example, the compact IRI form is used in two different ways. The first way, as shown with foaf:age declares both the IRI for the term (using short-form) as well as the @type associated with the term. The second way, only declares the @type associated with the term. In the second case, the JSON-LD processor will still derive the full IRI by looking up the foaf prefix in the context for foaf:homepage.

Full IRIs may also be used on the left-hand side of a context:

{
  "@context":
  {
    "foaf": "http://xmlns.com/foaf/0.1/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "name": "foaf:name",
    "foaf:age":
    {
      "@id": "foaf:age",
      "@type": "xsd:integer"
    },
    "http://xmlns.com/foaf/0.1/homepage":
    {
      "@type": "@id"
    }
  },
  ...
}

Note that in order for the full IRI to match above, the full IRI must also be used in the JSON-LD document. Also note that foaf:homepage will not use the { "@type": "@id" } declaration because foaf:homepage is not the same as http://xmlns.com/foaf/0.1/homepage. That is, a JSON-LD processor will use direct string comparison when looking up terms in a context before it applies the prefix lookup mechanism.

The only exception for using terms in the context is that they must not be used in a circular manner. That is, a definition of term-1 must not depend on the definition of term-2 if term-2 also depends on term-1. For example, the following context definition is illegal:

{
  "@context":
  {
    "term1": "term2:foo",
    "term2": "term1:bar"
  },
  ...
}

4.9 Embedding

Object embedding is a JSON-LD feature that allows an author to use the definition of JSON-LD objects as property values. This is a commonly used mechanism for creating a parent-child relationship between two subjects.

The example shows two subjects related by a property from the first subject:

{
...
  "name": "Manu Sporny",
  "knows":
  {
    "@type": "Person",
    "name": "Gregg Kellogg",
  }
...
}

An object definition, like the one used above, may be used as a JSON value at any point in JSON-LD.

4.10 Identifying Unlabeled Nodes

At times, it becomes necessary to be able to express information without being able to specify the subject. Typically, this type of node is called an unlabeled node or a blank node. In JSON-LD, unlabeled node identifiers are automatically created if a subject is not specified using the @id keyword. However, authors may provide identifiers for unlabeled nodes by using the special _ (underscore) prefix. This allows to reference the node locally within the document but not in an external document.

{
...
  "@id": "_:foo",
...
}

The example above would set the subject to _:foo, which can then be used later on in the JSON-LD markup to refer back to the unlabeled node. This practice, however, is usually frowned upon when generating Linked Data. If a developer finds that they refer to the unlabeled node more than once, they should consider naming the node using a resolve-able IRI.

4.11 Aliasing Keywords

JSON-LD allows all of the syntax keywords, except for @context, to be aliased. This feature allows more legacy JSON content to be supported by JSON-LD. It also allows developers to design domain-specific implementations using only the JSON-LD context.

{
  "@context":
  {
     "url": "@id",
     "a": "@type",
     "name": "http://schema.org/name"
  },
  "url": "http://example.com/about#gregg",
  "a": "http://schema.org/Person",
  "name": "Gregg Kellogg"
}

In the example above, the @id and @type keywords have been given the aliases url and a, respectively.

4.12 Expansion

The JSON-LD API [JSON-LD-API] defines an method for expanding a JSON-LD document. Expansion is the process of taking a JSON-LD document and applying a context such that all IRI, datatypes, and literal values are expanded so that the context is no longer necessary. JSON-LD document expansion is typically used as a part of Framing.

For example, assume the following JSON-LD input document:

{
   "@context":
   {
      "name": "http://xmlns.com/foaf/0.1/name",
      "homepage": {
        "@id": "http://xmlns.com/foaf/0.1/homepage",
        "@type", "@id"
      }
   },
   "name": "Manu Sporny",
   "homepage": "http://manu.sporny.org/"
}

Running the JSON-LD Expansion algorithm against the JSON-LD input document provided above would result in the following output:

{
   "http://xmlns.com/foaf/0.1/name": "Manu Sporny",
   "http://xmlns.com/foaf/0.1/homepage": {
      "@id": "http://manu.sporny.org/"
   }
}

4.13 Compaction

The JSON-LD API [JSON-LD-API] defines an method for compacting a JSON-LD document. Compaction is the process of taking a JSON-LD document and applying a context such that the most compact form of the document is generated. JSON is typically expressed in a very compact, key-value format. That is, full IRIs are rarely used as keys. At times, a JSON-LD document may be received that is not in its most compact form. JSON-LD, via the API, provides a way to compact a JSON-LD document.

For example, assume the following JSON-LD input document:

{
  "http://xmlns.com/foaf/0.1/name": "Manu Sporny",
  "http://xmlns.com/foaf/0.1/homepage": {
    "@id": "http://manu.sporny.org/"
  }
}

Additionally, assume the following developer-supplied JSON-LD context:

{
  "name": "http://xmlns.com/foaf/0.1/name",
  "homepage": {
    "@id": "http://xmlns.com/foaf/0.1/homepage",
    "@type": "@id"
  }
}

Running the JSON-LD Compaction algorithm given the context supplied above against the JSON-LD input document provided above would result in the following output:

{
  "@context": {
    "name": "http://xmlns.com/foaf/0.1/name",
    "homepage": {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    }
  },
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/"
}

The compaction algorithm also enables the developer to map any expanded format into an application-specific compacted format. While the context provided above mapped http://xmlns.com/foaf/0.1/name to name, it could have also mapped it to any arbitrary string provided by the developer.

4.14 Framing

The JSON-LD API [JSON-LD-API] defines an method for framing a JSON-LD document. This allows developers to query by example and force a specific tree layout to a JSON-LD document.

A JSON-LD document is a representation of a directed graph. A single directed graph can have many different serializations, each expressing exactly the same information. Developers typically work with trees, represented as JSON objects. While mapping a graph to a tree can be done, the layout of the end result must be specified in advance. A Frame can be used by a developer on a JSON-LD document to specify a deterministic layout for a graph.

Framing is the process of taking a JSON-LD document, which expresses a graph of information, and applying a specific graph layout (called a Frame).

The JSON-LD document below expresses a library, a book and a chapter:

{
  "@context": {
    "Book":         "http://example.org/vocab#Book",
    "Chapter":      "http://example.org/vocab#Chapter",
    "contains":     {
      "@id": "http://example.org/vocab#contains",
      "@type": "@id"
    },
    "creator":      "http://purl.org/dc/terms/creator",
    "description":  "http://purl.org/dc/terms/description",
    "Library":      "http://example.org/vocab#Library",
    "title":        "http://purl.org/dc/terms/title"
  },
  "@id":
  [{
    "@id": "http://example.com/library",
    "@type": "Library",
    "contains": "http://example.org/library/the-republic"
  },
  {
    "@id": "http://example.org/library/the-republic",
    "@type": "Book",
    "creator": "Plato",
    "title": "The Republic",
    "contains": "http://example.org/library/the-republic#introduction"
  },
  {
    "@id": "http://example.org/library/the-republic#introduction",
    "@type": "Chapter",
    "description": "An introductory chapter on The Republic.",
    "title": "The Introduction"
  }]
}

Developers typically like to operate on items in a hierarchical, tree-based fashion. Ideally, a developer would want the data above sorted into top-level libraries, then the books that are contained in each library, and then the chapters contained in each book. To achieve that layout, the developer can define the following frame:

{
  "@context": {
    "Book":         "http://example.org/vocab#Book",
    "Chapter":      "http://example.org/vocab#Chapter",
    "contains":     "http://example.org/vocab#contains",
    "creator":      "http://purl.org/dc/terms/creator"
    "description":  "http://purl.org/dc/terms/description"
    "Library":      "http://example.org/vocab#Library",
    "title":        "http://purl.org/dc/terms/title"
  },
  "@type": "Library",
  "contains": {
    "@type": "Book",
    "contains": {
      "@type": "Chapter"
    }
  }
}

When the framing algorithm is run against the previously defined JSON-LD document, paired with the frame above, the following JSON-LD document is the end result:

{
  "@context": {
    "Book":         "http://example.org/vocab#Book",
    "Chapter":      "http://example.org/vocab#Chapter",
    "contains":     "http://example.org/vocab#contains",
    "creator":      "http://purl.org/dc/terms/creator"
    "description":  "http://purl.org/dc/terms/description"
    "Library":      "http://example.org/vocab#Library",
    "title":        "http://purl.org/dc/terms/title"
  },
  "@id": "http://example.org/library",
  "@type": "Library",
  "contains": {
    "@id": "http://example.org/library/the-republic",
    "@type": "Book",
    "creator": "Plato",
    "title": "The Republic",
    "contains": {
      "@id": "http://example.org/library/the-republic#introduction",
      "@type": "Chapter",
      "description": "An introductory chapter on The Republic.",
      "title": "The Introduction"
    },
  },
}

4.15 Normalization

The JSON-LD API [JSON-LD-API] defines an method for normalizing a JSON-LD document. Normalization is the process of performing a deterministic transformation on a JSON-LD document resulting in a normalized representation.

Normalization is useful when comparing two graphs against one another, when generating a detailed list of differences between two graphs, and when generating a cryptographic digital signature for information contained in a graph or when generating a hash of the information contained in a graph.

The example below is an un-normalized JSON-LD document:

{
  "@context": {
    "name": "http://xmlns.com/foaf/0.1/name",
    "homepage": {
      "@id": "http://xmlns.com/foaf/0.1/homepage",
      "@type": "@id"
    },
    "xsd": "http://www.w3.org/2001/XMLSchema#"
  },
  "name": "Manu Sporny",
  "homepage": "http://manu.sporny.org/"
}

The example below is the normalized form of the JSON-LD document above:

[{
  "@id": "_:c14n0",
  "http://xmlns.com/foaf/0.1/homepage": {
    "@id": "http://manu.sporny.org/"
  },
  "http://xmlns.com/foaf/0.1/name": "Manu Sporny"
}]

Notice how all of the terms have been expanded and sorted in alphabetical order. Also, notice how the subject has been labeled with a named unlabeled node. Normalization ensures that any arbitrary graph containing exactly the same information would be normalized to exactly the same form shown above.